How to solve long multiplications?

Hi tehminanoor,

That's a great question! From my experience, it is very rare that you have to do a lot of tedious/complex calculating on the GMAT. If you find yourself having to do that, you may have missed the opportunity to use some more effective strategy for solving the problem.

Here are a few tips that may be helpful:
1) Learn the mental math tricks. A quick google search for "mental math tricks" will give you tons of helpful resources.
2) One important thing about the GMAT structure is that you don't get any points for precise accuracy. You just need to solve enough of the problem to choose the correct answer out of the five given to you (or eliminate 4 of them). Therefore, you can employ tactics like...

-estimation/ballparking - round numbers up or down and then solve the problem to get a rough idea of what the answer should look like. This will often help you eliminate answer choices or choose between a couple remaining options.
-plugging in answers - exactly what it sounds like, take the answer and see if you can plug it in as a variable
-discover a math property - sometimes there is a math property out there but in the pressure of the test you don't know it or remember it. Do some quick experiment to see if you can identify a pattern
-patterns - similarly if you know of a pattern already, then that might help you quickly do the math. For example: in three digit numbers, if you add each digit and the sum is a multiple of 3, then the full number is also a multiple of 3.
-solve enough of the problem to where you identify the right answer. For example, in a complex multiplication problem, once you figure out that the ones digit = 5, you can eliminate all of the answer choices that don't have a 5 in the ones digit.

These and other tricks may help you from having to do tedious calculations. Hope you find it helpful!